Curcumin is a yellow-orange compound derived from the root of Curcuma longa L, that has been used as a medicine, spice and coloring agent ( GRAS). It is a pharmacologically important natural product. So far several studies have failed to demonstrate the curcumin toxicity in a number of cell culture and whole animal studies. Alcohol extract and active ingredients from this rhizome have, however, been reported to have (gram positive) bactericidal effects. In our preliminary study we found potent phototoxic effects for curcumin in both gram-positive and gram-negative bacteria in micromolar amounts, levels hundreds of times lower than previously reported without light. We believe that oxygen is involved in the phototoxicity of curcumin, at least for the gram-positive organisms. The mechanism(s) by which curcumin kills these cells in the presence of light, and the role of oxygen in this toxic effect, are yet to be elucidated. Our proposal deals with addressing the photodynamic action of curcumin via oxygen. Curcumin is also reported to have other isomers. We plan to isolate the isomers from the natural product and identify the basic component responsible for the photodynamic activity. Our goal is to identify the cause of the phototoxicity, whether it is due to the generation of singlet oxygen, superoxide generation or reaction product of curcumin itself. Singlet oxygen and superoxide methods developed in our laboratory will be used to investigate the mechanisms. Electrochemical, spectral and photochemical methods will be used to characterize the isomers and their activities.